Page 146 - Biodegradable Polyesters

124 5 Crystallization of Poly(lactic acid)

triblock copolymers [92], branched PLAs on polyglycidol [93], star-shaped PLAs
[94], and three-armed PLAs [95]. Among them, the stereoblock PLAs showed
excellent efficiency in stereocomplex formation [87, 96, 97].
Recently, the polymorphic phase transition from the PLLA and PDLA
homo-crystals to PLLA/PDLA stereocomplex crystals upon heating has been
investigated by temperature-variable wide- and small-angle X-ray diffraction
and infrared spectroscopy [84, 98]. It was found that during heating of an
oriented PLLA/PDLA blend, prepared by stretching the quenched blend to
∘
′
four times its initial length at a temperature of 85 C, and consisting of α -form
′
homo-crystals and small amount of stereocomplex crystals, the α -form homo-
∘
crystals first transform into their α-counterparts at 120–160 C, followed by a
∘
further transition into the stereocomplex crystals at 160–210 C, as shown in
Figure 5.14 [84]. The polymorphic transition from the α-form homo-crystals
to the PLA stereocomplex crystals occurs via a solid–melt–solid transfor-
mation [84].

200/110 ×10
Heating
α′
40 °C
α′
60 °C
80 °C
100 °C
Intensity 120 °C
140 °C
160 °C
α
α
180 °C
200 °C
β c
210 °C β c
110
5 15 25 35 25 30 35 40
Scattering angle 2θ (°)

Figure 5.14 (a) WAXS profiles of a stretched correspond to the scattering character-
PLLA/PDLA blend recorded upon heat- istics of PLA stereocomplex crystals. (b)
′
′
ing. α , α,and β denote the α -, α-form Enlarged profiles in the higher-diffraction-
c
homo-crystals, and stereocomplex crys- angle region. The dashed lines indicate
tals, respectively. The strong peaks around the characteristic peaks of the PLA α-
∘
∘
2θ= 16.5 and 19.0 are the charac- form. Adapted with permission from Ref.
′
teristic reflections of the α -and α-form [84], Copyright (2007) American Chemical
homo-crystals of PLA. The three peaks Society.
∘
∘
around 2θ= 11.1 , 20.0 , and 23.2 ∘